This invention relates to the field of interface circuitry. In particular, this invention is drawn to beta compensation of interface circuitry between an integrated circuit and circuitry external to the integrated circuit.
Complementary metal oxide semiconductor (CMOS) fabricated integrated circuits are popular due to well known manufacturing requirements, availability of design tools, and cost of production. Although CMOS integrated circuits may be fabricated for either high or low voltage operation, data manipulation functions such as signal processing, computational, storage, and other data handling functions are typically implemented with low voltage processes due to cost considerations. CMOS integrated circuit fabricated with low voltage processes are not well suited for direct interfacing with higher voltage circuits.
Interface circuitry can be used to interface the CMOS circuitry with higher voltage circuitry. Signals communicated through the interface circuitry are subject to modification or distortion in accordance with the transfer characteristic of the interface circuitry. Bipolar junction transistors (BJT) are frequently used to interface CMOS circuitry with higher voltage circuitry. The current transfer characteristic between the collector and emitter of a BJT is referred to as xcex1 (alpha) which is itself a function of the transistor common emitter current gain, xcex2 (beta).
One disadvantage of BJT interface circuitry is that xcex2 may vary widely between transistors of similar make and manufacture. As a result, the transfer functions of interface circuitry may vary considerably. Although compensation circuitry may be introduced to minimize variations between individual interface circuitry, such compensation circuitry typically requires additional external components that tend to increase the cost of manufacturing and assembling a system with the integrated circuit and the interface circuitry.
An apparatus for interfacing an integrated circuit with external circuitry includes a transistor having an emitter coupled to a node for receiving a control current (IP) from the integrated circuit. A beta compensator disposed within the integrated circuit provides a compensating current (IM) to the node. The compensating current IM is proportional to a base current (IB) of the transistor.
In one embodiment, the transistor is a bipolar junction transistor. The transistor collector is coupled to the external circuitry. When the proportionality factor between IM and IB is unity, the transistor collector current (IC) is substantially the same as the control current (IC=IP).
A method of interfacing the integrated circuit with external circuitry includes the step of providing a control current (IP) from the integrated circuit to a node. A transistor emitter is coupled to the node. A compensating current (IM) is provided to the node in response to the transistor base current (IB). The net node current provided to the emitter is IP+IB.
In one embodiment, the method and apparatus are applied to a linefeed driver of a subscriber line interface circuit.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.